(1) Field of the Invention
This invention relates to an electrical unit containing heat-generating components inside.
(2) Description of the Related Art
Usually, an internal circuit, such as an electrical unit 100 as shown in FIG. 9, is installed in an electric connection box.
The electrical unit 100 includes a circuit board 10, heat-generating components 20 soldered to a wiring pattern (not shown) on the circuit board 10, and four terminal-attached covers 130 attached to the circuit board 10.
On the circuit board 10, there are formed slits 11 and V-shape cuts 12.
Each heat-generating component, 20 is a component such as a relay 21, a positive temperature coefficient (hereinafter, PTC) element 22 and a transistor 23. The PTC element increases its own electrical resistance to prevent a current from flowing into the circuit when an overcurrent takes place in the circuit.
The terminal-attached cover 130 contains a base 131 made of synthetic resin, legs 132 for screwing the terminal-attached cover 130 on the base 131, which are integrally molded with the base 131, and the terminals 133 that penetrate the base 131 and stick out up and down. The heat-generating components 20 electrically connected to the circuit board 10 are covered with the terminal-attached cover 130 that is screwed on the circuit board 10 by screws 60 inserted into the legs 132.
After the heat-generating components 20 are placed on the circuit board 10, four terminal-attached covers 130 are mounted on the circuit board 10. The heat-generating components 20 are soldered on the circuit board 10 by using an automatic soldering machine, then the circuit board 10 is divided into four parts by cutting along the slits 11 and V-shape cuts 12. Thus, four identical electrical units are produced at a time.
A design is disclosed in Japanese Utility Model Application Laid-Open No. S61-93719, in which a circuit board 141 is covered with a cover member 140 (corresponding to the above terminal-attached cover 130) made of flameproof resin as shown in FIG. 10. A part 143 installs an electrical unit 100' therein.
An invention is disclosed in Japanese Patent Application Laid-Open No. H3-235610, in which the cover member 140 is made of a material having high thermal conductivity prepared by mixing an inorganic material into a resin. A large amount of (50 wt % or more) glass fibers is mixed into polybutylene terphthalate (hereinafter, PBT) in order to increase the thermal conductivity.
If a heat-generating component 20 catches fire caused by a heat generation, there is a danger that the terminal-attached covers 130 might burn if the resin that is used to make the terminal-attached covers 130 is not flameproof, causing deterioration in quality of the electrical unit 100. When the heat-generating component 142 catches fire and falls down to be in contact with another heat-generating component 142', there is a danger that the heat-generating component 142' also might catch fire if no measure to prevent the heat-generating component 142 shown in FIG. 10 from falling down is taken, causing deterioration in quality of the electrical unit 100.
When the cover member 140 is made of a material having high thermal conductivity prepared by mixing a large amount of an inorganic material into a resin, there is no problem with respect to the material itself of the cover member 140, however a cost of molding the cover member 140 increases since a molding condition thereof becomes severe.